The history of the polypropylene polymers started in 1954 with the researchers Giulio Natta and Karl Ziegler. This plastic material produced for the first time in the Italian industry by Montecatini (then Montedison) has known a huge success since the very beginning. Among the many applications of polypropylene, today this product has become increasingly more important for the food packaging industry and particularly for disposable cups.
Before the thermoforming process, the material is extruded, calendered, cooled and transformed into rolls. The thickness of the sheet may range from 0.2 mm. to 2 mm.
During the production process, the material is being unwind and then fed into the thermoforming machine through guides with toothed chains passing the infrared heating oven in order to obtain the necessary softness before entering the forming station.
During the transformation of the Polypropylene, it is necessary to take into account the two main mechanical characteristics of this polymer such as the:
- Coefficient of thermal expansion
- Specific heat coefficient of the material
The polypropylene has a thermal expansion coefficient three times bigger and a specific temperature two times higher than APET. Consequently, to process the polypropylene, it is important to pay attention to the heating process and slackening of the internal tensions of the material in order to obtain a stable and qualitative molding process. As we deal with a material coming from an extrusion and calendering process, the polypropylene re-heated for a second time, has the tendency to slack. The material, which is retained in the chains teeth, frequently may show undulations that may be difficult to uneven and therefore creating problems in distributing the material during the forming process affecting the esthetics of the final product.
In order to avoid these drawbacks, it is necessary to preheat the foil to allow the "stress relaxation" of the material before placing it on the transport chains of the thermoforming machine and consequently obtaining a homogenous temperature in the core of the sheet through a slow but uniform heating process. The stress relaxation is obtained by pre-heating the material gradually and under "free" conditions before the thermoforming process as the polypropylene tends to distress naturally during heating when it is not being blocked in the chains.
For this reason, special pre-heaters have been studied and the market offers different solutions. Since the mid-eighties, WM Wrapping Machinery SA has performed a number of tests developing various alternatives (see picture 1) such as infra-red radiation systems, contact by rollers etc.
The best final solution that ensured the greatest benefit was the oven with hot air circulation (see pictures 2 and 3).
A special pre-heather has been developed which consists of an insulated structure and mobile roller located on the inside which enables various meters of foil being treated continuously during the pre-heating phase before being entered into the thermoforming machine. Inside the heating chamber, the air is heated and temperature-controlled to a maximum of 130 ºC. A centrifugal fan with an internal high performance auger and impeller with reversed blades, generates an upward airflow. In addition, a special device called "Full air delivery" is installed on the floor, inside the oven, in order to ensure a correct distribution of the hot air flow.
The heating takes place through a group of electric heat-controlled resistances and is controlled by a dedicated probe inside the oven. This PH pre-heating unit is totally autonomous, equipped with a control panel and can be installed easily between the reel-unwinding device and the thermoforming unit. The PH oven may be installed on existing machines at Customers factory. (See picture 5)
WM is offering the PH 920 and PH 1100 models which are differing in power and maximum size width of the foil, ranging from 920mm to 1,100 mm respectively. These pre-heaters have evolved significantly over the last few years and the last generation of the PH series are the result of the experiences and tests made over the years at WM factory. As already mentioned before, the insulation element is of high importance ensuring a maximum performance with the highest possible energy savings. It's important to highlight that the standard ovens chamber insulation has a thickness of 80 mm.
It is also important to say that the feeding rolls, which consist of a double rollers unit, are aligned in a upper vertical position when entering the material. This allows a quick and easy feeding of the foil by the operator, then, one of the two rolls group moves downward in order to position the material with the best possible distribution in the heating chamber (see picture 4).
The feedback we receive from our customers who already use this kind of pre-heaters it is absolutely positive and these experiences recommend the usage of this device during the processing of polypropylene materials ensuring high performance and top quality finished products.
The Federal's pistol slid into its holster and his sabre flashed out. He threw its curved point up in a splendid salute. Ferry saluted with his straight blade. Then both swords rang back into their scabbards, and Jewett whirled away toward his column. For a moment we lingered, then faced to the left, trotted, galloped. Over the fence and into the road went he--went I. Down it, as we crossed, the blue column was just moving again. Then the woods on the south swallowed us up. "All right, sir." that before. Meanwhile the parallelism between Thought and Extension was not exhausted by the identification just analysed. Extension was not only a series of movements; it still remained an expression for co-existence and adjacency.412 Spinoza, therefore, felt himself obliged to supply Thought with a correspondingly continuous quality. It is here that his chief originality lies, here that he has been most closely followed by the philosophy of our own time. Mind, he declares, is an attribute everywhere accompanying matter, co-extensive and co-infinite with space. Our own animation is the sum or the resultant of an animation clinging to every particle that enters into the composition of our bodies. When our thoughts are affected by an external impulse, to suppose that this impulse proceeds from anything material is a delusion; it is produced by the mind belonging to the body which acts on our body; although in what sense this process is to be understood remains a mystery. Spinoza has clearly explained the doctrine of animal automatism, and shown it to be perfectly conceivable;569 but he has entirely omitted to explain how the parallel influence of one thought (or feeling) on another is to be understood; for although this too is spoken of as a causal relation, it seems to be quite different from the logical concatenation described as the infinite intellect of God; and to suppose that idea follows from idea like movement from movement would amount to a complete materialisation of mind; while our philosopher would certainly have repudiated Mr. Shadworth Hodgson’s theory, that states of consciousness are only connected through their extended substratum, as the segments of a mosaic picture are held together by the underlying surface of masonry. Nor can we admit that Spinoza entertained the theory, now so popular, according to which extension and consciousness are merely different aspects of a single reality. For this would imply that the substance which they manifest had an existence of its own apart from its attributes; whereas Spinoza makes it consist of the attributes, that is to say, identifies it with their totality. We are forced, then, to conclude that the proposition declaring thought and extension to be the same thing570 has no413 other meaning than that they are connected by the double analogy which we have endeavoured to explain. “Oh, he’s welcome to it,” Sandy cried. “I’ve turned over a new leaf!” “But—are they?” He pulled a wire out a trifle from the sheath. The sergeant spent more time upon the oaths with which he embellished the counter-question as to how he should know anything about it, than would have been consumed in a civil explanation. "Doctor, he can't die. He mustn't die," said Shorty in agony. "The regiment can't spare him. He's the best soldier in it, and he's my pardner." AND ENTERS UPON HIS PARENTAL RELATIONS TO LITTLE PETE SKIDMORE. "Holy smoke, look there," gasped Monty Scruggs, as a company of rebel cavalry came tearing over the hill in front, to the assistance of their comrades. "That's about it," answered Shorty disdainfully. "That's the way with all cavalry, dad-burn 'em. They're like a passel o' fice pups. They're all yelp and bark, and howl and showin' o' teeth. They're jest goin' to tear you to pieces. But when you pick up a stone or a club, or git ready to give 'em a good kick they're gone, the devil knows where. They're only an aggravation. You never kin do nothin' with 'em, and they kin do nothin' with you. I never kin understand why God Almighty wasted his time in makin' cavalry of any kind, Yank or rebel. All our own cavalry's good for is to steal whisky and chickens from honest soldiers of the infantry. The infantry's the only thing. It's like the big dog that comes up without any special remarks, and sets his teeth in the other dog. The thing only ends when one dog or the other is badly whipped and somethin's bin accomplished." "Mmm." Reuben started out of the half-waking state into which he had fallen. It was late in the afternoon, the sunlight had gone, and a wintry twilight crept up the wall. Maude the dairy-woman was looking in at the door. It was about a month subsequent to this, that one morning, as Turner was making the anvil ring with the ponderous strokes of his hammer, two retainers from the castle entered the shed, and delivered an order from De Boteler for his immediate attendance. Wat laid the hammer on the anvil, and, passing the back of his right hand across his forehead, to clear away the large drops that stood there, looked with a kind of smile at the men as he said, HoME泷泽萝拉四部和集下载
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